RIO DE JANEIRO (AP) — Cristiano Ronaldo is ready to lead Portugal against Germany in one of the biggest games of the World Cup's first week, saying his sore knee won't be a problem.

The Real Madrid forward, arguably the most recognizable player in the sport, said he wouldn't risk his career by going into such a tough match with a serious injury. The match at Salvador features two of the top teams in the world, with Germany ranked No. 2 and Portugal at No. 4.

In the other Group G match Monday, Jurgen Klinsmann's U.S. team is aiming to reverse its record against Ghana, which has ended the Americans' run at the last two World Cups. Nigeria and Iran meet in Group F.

Things to watch for Monday:

NOT ALL ABOUT THE KNEE: Ronaldo has been bothered by a left knee problem in recent weeks, but says he'll be "100 percent" to take on Germany. He knows Portugal hasn't fared well in recent matches against Germany at major events — the Germans beat Portugal for third place at the 2006 World Cup, and have won their head-to-heads at the last two European championships.

But the world player of the year is confident some club form could be a good omen. He helped Real Madrid beat Bayern Munich in the Champions League semifinals and then clinched the title.

"Maybe this will be the year to change the history and Monday would be the best day for it to happen," said Ronaldo, Portugal's all-time leading scorer. "It's a new tournament, new players, our team is good, we have confidence."

Ronaldo's mantra this week has been "one player is not a team" — but Portugal does tend to struggle when he's not available. Joao Moutinho will have to create opportunities in midfield and Manchester United winger Nani needs to find form and fitness to have any impact on the tournament.

Germany coach Joachim Loew was keeping his starting lineup secret, but injuries will force some changes. Sami Khedira, goalkeeper Manuel Neuer, captain Philipp Lahm and Bastian Schweinsteiger are all coming back from injuries. Veteran forward Lukas Podolski could play an important role in what will be his third World Cup.

Venue: Salvador. Kickoff: 9 a.m. PT (TV on ESPN).

DRAINING CONDITIONS: A two-day downpour has flooded some streets and caused mudslides in Natal. The forecast is for more unsettled weather when the Americans open their campaign against Ghana, which knocked them out in the second round in 2010 and ended their run in the group stage in '06.

The U.S. has never lost to a team three straight times at the World Cup and Ghana captain Asamoah Gyan, who scored the goal that clinched the win over the Americans four years ago, says "It's going to be like they're coming for revenge."

The spotlight will be on the U.S. midfield, where Kyle Beckerman, Jermaine Jones and Michael Bradley will likely play key roles feeding Clint Dempsey and Jozy Altidore in the absence of World Cup veteran Landon Donovan, who was cut from the squad last month.

Donovan will be working as a TV analyst.

Ghana is coming off a 4-0 win over 2002 semifinalist South Korea in its last warmup match. Gyan and midfielder Sulley Muntari, playing in their third World Cups, and Michael Essien and Kevin-Prince Boateng — both returning after long breaks from the international arena — have crucial leadership roles on a squad with 16 World Cup rookies.

Venue: Natal. Kickoff: 3 p.m. PT (TV on ESPN).

WINNING COULD MEAN EVERYTHING: Nigeria is aiming to shake off its underachiever tag. To do that, the African champion has to start by winning its first World Cup match since 1998. First up is Iran, another country that hasn't won a World Cup game since 16 years ago, when it had a big win over the United States in France. Two-time champion Argentina had a 2-1 win over newcomer Bosnia-Herzegovina and should dominate Group F, where there is no clear favorite for the No. 2 spot.

Nigeria's hopes could rest on the form and fitness of striker Emmanuel Emenike and the support he gets from Chelsea midfielder John Obi Mikel.

The 33-year-old Javad Nekounam will lead Iran with reinforcement coming from Reza "Gucci" Ghoochannejhad and Ashkan Dejagah, who turned down chances to play for European countries so they can represent the country of their birth.